This invention relates to ultrashort optical pulse modulating equipment which permits high multiplexing of optical pulses and, more particularly, to ultrashort optical pulse modulating equipment which affords reduction of optical power loss and of the number of optical components involved.
A high-speed optical cell (packet) switch (Optical ATM: Optical Asynchronous Transfer Mode) is now receiving attention as a large-capacity optical switch system of the next generation.
FIG. 1 shows an optical signal multiplexer for use in such a high-speed optical cell switch. In FIG. 1 ultrashort optical pulse modulating equipments 21.sub.1 through 21.sub.N yield at regular intervals optical packet signals P.sub.11, P.sub.12, . . . ; P.sub.21, P.sub.22, . . . ; . . . ; P.sub.N1, P.sub.N2, . . . , respectively. The optical packet signals are each composed of a string of a predetermined number of bits, for example, m information bits or optical pulses. The optical packet signals are applied to, for instance, optical fiber delay lines 22.sub.1 through 22.sub.N, respectively, by which they are delayed relative to one another for a fixed time Td a little longer than the packet length as shown at rows A to D in FIG. 2. The optical packet signals of the respective channels, output from the optical fiber delay lines 22.sub.1 to 22.sub.N, are multiplexed by an optical coupler 23, from which are provided such multiplexed optical packet signals P.sub.11, P.sub.21, . . . , P.sub.N1 , P.sub.12, P.sub.22, . . . as shown at row E in FIG. 2.
The ultrashort optical pulse modulating equipments 21.sub.1 through 21.sub.N each converts an input electrical signal into an optical packet signal. It is desired, for large-capacity optical switching, that the optical pulse interval in each packet be minimized (about the same as the optical pulse width, for example) to reduce the packet length to thereby increase the number of multiplexing channels. To meet this requirement, there has been proposed such optical pulse modulating equipment as shown in FIG. 3, which produces a modulated optical pulse train of a very short pulse interval. A pulse generator 10, which is supplied with an input electrical signal S composed of information bits of a period T as shown at row A in FIG. 4, regenerates clock signals from the information bits and generates drive pulses Dp of a period mT in synchronism with the clock signals as depicted at row B in FIG. 4. A laser 11 is driven by the drive pulses Dp to generate optical pulses Lp (the same as those shown at row B in FIG. 4), which are applied to an optical splitter 12. The optical splitter 12 splits each optical pulse into channels of the same number m as the bits of each packet, through which the optical pulses are applied to external modulators 13.sub.1 to 13.sub.m, respectively.
On the other hand, the information bits of the input electrical signal S are sequentially applied to a shift register 16 having shift stages of the same number as the bits of one packet (i.e. m stages). Upon each application of the information bits of one packet to the shift register 16, outputs of its respective stages are simultaneously provided as modulation signals to the corresponding external modulators 13.sub.1 to 13.sub.m in synchronism with the optical pulses Lp. The external modulators 13.sub.1 through 13.sub.m are each formed by an optical switch, for instance, which modulates the optical pulse in accordance with the modulation signal by passing therethrough or cutting off the optical pulse, depending on whether the modulation signal is high-level or low-level. Assuming, for the sake of brevity, that the modulation signals applied to the external modulators 13.sub.1 through 13.sub.m are all high-level, the modulated optical pulses (all high-level) are applied to optical fiber delay lines 14.sub.1 through 14.sub.m of the respective channels, by which they are sequentially delayed for a time .tau. relative to one another as shown at rows C to F in FIG. 4. The optical pulses thus delayed are multiplexed by an optical coupler 15 into a string of m optical pulses of a constant period .tau. as depicted at row G in FIG. 4. The delay time .tau. is set to, for example, about twice the width of each ultrashort optical pulse Lp. Letting the length of the shortest optical fiber delay line 14.sub.1 be represented by L, the lengths of the optical fiber delay lines 14.sub.1 to 14.sub.m for providing such a relative delay .tau. are L, .tau..multidot.C/n.sub.f, 2.tau..multidot.C/n.sub.f +L, . . . , (m-1).tau..multidot.C/n.sub.f +L, respectively, where C is the velocity of light in a vacuum and n.sub.f is the refractive index of the fiber core.
As will be appreciated from comparison of rows A and G in FIG. 4, the train of pulses of the input electrical signal S, which are of the period T, is converted by the optical pulse modulating equipment of FIG. 3 into the train of optical pulses of the period .tau., whereby it is output as an optical packet signal of a packet length m.tau. compressed from the packet length mT of the input electrical signal S. The optical packet signal thus compressed is multiplexed with optical packet signals from other optical pulse modulating equipment as referred to previously in respect of FIGS. 1 and 2. Incidentally, in the optical pulse modulating equipment shown in FIG. 3, since the output optical pulses from the laser 11 are split by the optical splitter 12 into m channels, the power of the optical pulse in each channel is reduced to 1/m the input optical pulse, and consequently, the power level of each optical pulse of the optical pulse string output from the optical coupler 15 is also reduced to 1/m or less. A similar loss also occurs in the optical coupler 15. Moreover, assuming that the number m of bits of each optical packet is m=2.sup.9 =512, it will be necessary to employ 512 external modulators 13.sub.1 to 13.sub.m and 512 optical fiber delay lines 14.sub.1 to 14.sub.m, and consequently, the number of components used is very large, resulting in the optical pulse modulating equipment inevitably becoming bulky. In the case of m=2.sup.9, the optical splitter 12 calls for a tree structure involving 2.sup.9 -1=511 optical splitters (hereinafter referred to as 1:2 optical splitters) each of which splits input light into two, and the optical coupler 15 also calls for a similar tree structure. Letting m=2.sup.n, where n is a positive integer, the number of optical elements needed for forming the optical splitter 12 and the optical coupler 15 is 2.times.(2.sup.n -1)=2.sup.n+1 -2. The larger the numbers of 1:2 optical splitters and 2:1 optical couplers, the more the loss of optical power. Hence, such a large number of optical elements used is not preferable.